1. Field of the Invention
The present invention relates to a fabrication method for heterojunction bipolar transistor (HBT), and more particularly to a fabrication method for heterojunction bipolar transistor (HBT) emitter/emitter window by a reverse-tone approach.
2. Description of the Related Art
Transistors are multi-electrode semiconductor devices in which the current flowing between two specified electrodes is controlled or modulated by the voltage applied at a third (control) electrode. Transistors fall into two major classes: the bipolar junction transistor (BJT) and the field-effect transistor (FET). BJTs were derived from the point-contact transistor, which was invented at Bell Telephone Laboratories in 1947 by Bardeen, Brattain, and Shockley. BJTs comprise two p-n junctions placed back-to-back in close proximity to each other, with one of the regions common to both junctions. This forms either a p-n-p or n-p-n transistor comprising three regions—emitter, base and collector. The BJT utilizes the flow of both electrons and holes across the p-n junctions for its electrical behavior. That is, the current flow through the emitter and collector electrodes is controlled by the voltage across the base-emitter p-n junction.
In normal (or forward active) operation of a BJT, the base-emitter p-n junction is forward biased and the base-collector junction is reverse biased. Majority-carrier current flows across the forward-biased emitter-base junction. The emitter is much more heavily doped than the base region, so that most of the total current flow across the base-emitter junction consists of majority carriers from the emitter injected into the base. These injected carriers become minority carriers in the base region, and will tend to recombine. Such recombination is minimized by making the base region very narrow, so that the injected carriers can diffuse across the base to the reverse-biased base-collector junction, where they are swept across the junction into the collector, to appear in the outside circuit as the collector current. The magnitude of this collector current depends on the number of majority carriers injected into the base from the emitter, and thus current is controlled by the base-emitter p-n junction voltage. The output (collector) current is therefore controlled by the input (base-emitter) voltage, and the output circuit of the transistor can be modeled as a voltage-controlled current source (dependent sources), while the input circuit looks like a p-n junction diode.
In principle, the transistor can be operated in reverse active mode by reversing the connections. However, in practice, the transistor is not completely symmetrical. That is, the emitter is very heavily doped to maximize emitter injection, and the collector is relatively lightly doped so that it can accommodate large voltage swings across its reverse-biased junction. While the electrical characteristics are similar in appearance, the forward characteristics show much greater gain, as expected.
If both junctions are reverse biased, the transistor behaves like an open switch, with only the p-n junction reverse leakage currents flowing. If both junctions are forward biased, there is injection of carriers into the base region from both sides, and a low resistance is presented to current flow in either direction: the transistor behaves like a closed switch, and the base stores the injected charge.
BJTs can be used to provide linear voltage and current amplification: small variations of the base-emitter voltage and hence the base current at the input terminal result in large variations of the output collector current. Since the transistor output has the appearance of a current source, the collector can drive a load resistance and develop an output voltage across this resistance (within the limits of the supply voltage). The transistor can also be used as a switch in digital logic and power switching applications, switching from a high-impedance ‘off’ state in cut-off, to a low-impedance ‘on’ state in saturation. In practice, full saturation conditions of base-collector forward biased are generally avoided, to limit the carrier storage in the base and reduce the switching time. Such BJTs find application in analog and digital circuits and integrated circuits, at all frequencies from audio to radio frequency. For higher frequencies, such as microwave applications, heterojunction bipolar transistors (HBTs) are used.
HBTs are bipolar junction transistor which incorporate a wide band gap emitter, where the emitter-base junction is a heterojunction between semiconductors of different energy band gaps. The following are typical materials for HBTs: aluminum-gallium-arsenide (AlGaAs) (emitter)/gallium-arsenide(GaAs) (base); aluminum-indium-arsenide (AlInAs)/indiumgallium-arsenide (InGaAs); Si/silicon-germanium (SiGe); and indium-gallium-phosphide (InGaP)/GaAs; indium-phosphide (InP)/InGaAs.
However, conventional HBTs and the forming process thereof have several drawbacks. FIG. 1A to FIG. 1C show a conventional HBT process. Referring to FIG. 1A, a substrate 102 having a collector, a dielectric 104, a layer 106, a base layer 108, a dielectric layer 110, a hard mask layer 112, a BARC layer 114 and a photoresist layer 116 with a window pattern is shown. Then the BARC layer 114, the hard mask layer 112 and the dielectric layer 110 are etched by reactive ion etching (RIE) and wet dip processes to expose the base layer 108 as shown in FIG. 1B. Finally, an emitter 118 is formed and an extrinsic base region 120 is formed by an ion implantation process.
The above-mentioned process has several drawbacks. Firstly, the critical dimension control of the emitter window is hard and tough because the emitter window pattern is formed by a photo mask with a window/hole pattern which is hardly shrink, especially when the critical dimension control shrinks toward to 0.18 micron generation. Moreover, the emitter window pattern formed by the window/hole pattern tends to enlarge in a photolithography process. Furthermore, the emitter window formed by reactive ion etching and wet dipping the BARC layer 114, the hard mask layer 112 and the dielectric layer 110 will be further enlarged. Defects such as voids induced by RIE and wet dip will be formed between the emitter 118 and the base layer 108 after the emitter 118 is formed on the base layer 108.
Thus it is necessary to provide a new method to resolve the drawbacks set forth. It is towards those goals that the present invention is specifically directed.